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Authorities were trying to evacuate the World Trade Centre when its towers collapsed within minutes of each other. About 50,000 people work there and tens of thousands visit it every day.

Surrounding buildings and thousands of people in the streets below were covered in a thick coating of dust thrown up by the collapsed towers.

Later a third building at the centre collapsed. Among the many missing people were 200 firemen.

"I would not want to say what the death toll could be," Rudolph Giuliani, the mayor, said. "It will be a horrible number. I saw people dropping out of windows."

Senior officers and hundreds of soldiers were feared dead in the Pentagon attack, which was carried out with a Boeing 757 hijacked shortly after it took off from Washington's Dulles airport on its way to Los Angeles.

The rest of the Pentagon was immediately evacuated, but firemen were unable to reach the crash area because of the intense heat from the crash.

The National Guard was mobilised in Washington and New York and forces in the American capital were put on Threat Level Delta, the equivalent of a war alert.

In the skies above both cities, the only planes to be seen were F-16 and F-15 fighters scrambled for fear of yet more attacks.

At Norfolk, Virginia, one of main naval bases, five warships put to sea to protect the Atlantic coast and two aircraft carriers headed for New York.

President Bush, who was flown to the safety of a bunker in Nebraska, called for calm and promised revenge on the perpetrators. He was due to address the nation on television last night after returning to the White House.

Mr Bush, who was told of the attacks as he read to primary school children in Florida, said: "Freedom itself was attacked this morning by a faceless coward and freedom will be defended.

"Make no mistake, the United States will hunt down and punish those responsible for these cowardly acts."

Dick Cheney, the Vice President, and Gen Colin Powell, the Secretary of State, were collected by the Secret Service and taken back to heavily guarded quarters in the capital. Members of Congress spoke of the outrages against their nation as an act of war.

The extreme Islamic Taliban movement in Afghanistan, which has provided a home to bin Laden, denied responsibility.

Abdul Salam Zaeef, the Taliban ambassador to Pakistan, condemned the attacks and said he hoped that the perpetrators would quickly be brought to justice.

"Osama is only a person," he said. "He does not have the facilities to carry out such activities. We want to tell the American people that Afghanistan feels their pain."

The attack caused jubilation in Iraq and among some radical groups in the Middle East, which saw it as revenge for America's support for Israel.

Islamic Jihad, responsible for many of the recent suicide bombings in Israel, said: "The United States has provided a cover for these Zionist crimes. "We are happy to see America suffer the pain and bitterness that our people feel."

World leaders condemned the outrages and offered help to America. The Queen, who is at Balmoral, sent a message of sympathy to President Bush.

She said: "It is with growing disbelief and total shock that I am learning of the terrorist outrages in New York and Washington today.

"On behalf of the British people, Mr President, may I express my heartfelt sympathy to the very many bereaved and injured and our admiration for those who are now trying to cope with these unfolding tragedies. Our thoughts and prayers are with you all."

Tony Blair said that Britain stood "shoulder to shoulder" with the United States. He announced a series of security measures, including a halt to all flights across central London.

Chancellor Schröder of Germany called the attack a "declaration of war".

Dr Henry Kissinger, who was the secretary of state under President Nixon, said: "This is comparable to Pearl Harbor and we must have the same response and the people who did it must have the same end as the people who attacked Pearl Harbor."

Within hours of the attacks many government buildings throughout America, including the White House, had been closed to the public.

Many familiar landmarks of American life closed also. They included the Empire State Building, the Washington monument, Disneyland, the Kennedy space centre, the Sears tower in Chicago and the Space Needle in Seattle.

Last night the Foreign Office warned Britons in Muslim countries of the risk of further violence after the attacks on America and of "heightened tension and disruption worldwide".